The upgrade didn’t cause my problem, but the problem is the same. I changed themes (2 different themes on 2 different websites) and both times the featured images no longer display. I tried several different thumbnail regeneration plugins, but they didn’t help. I can manually set the featured image for each post, but on one of the blogs I have over 1000 posts. There must be some way to automatically reset all the featured images, but I can’t find it. Help!
The only plugin is Akismet and the theme is updated to 3.4.1.